This invention relates to inverter apparatus, more particularly inverter apparatus for producing a variable voltage, variable frequency polyphase alternating current output suitable for driving an alternating current motor from a direct current source.
FIG. 1 shows the main circuit of prior art inverter apparatus capable of controlling current by the inverter apparatus itself. The inverter apparatus comprises 6 sets of chopper circuits CH1 through CH6 which are constituted by 6 main thyristers M1 through M6, auxiliary thyristors A1 through A6 of the reverse conduction type and utilized for commutation, commutating reactors L1 through L6, and commutating capacitors C1 through C6. Main thyristors M1 and M4, and smoothing reactors LD1 and LD4 are connected in series across positive and negative bus lines of a DC source E. In the same manner, main thyristors M3 and M6 and smoothing reactors LD3 and LD6 are connected in series across the positive and negative bus lines, and main thyristors M5 and M2 and smoothing reactors LD5 and LD2 are also connected in series across the positive and negative bus lines. Phase selection and circulation thyristors S1 through S6 are connected between respective chopper circuits and the positive and negative bus lines. The AC output is derived out from output terminals U, V and W between smoothing reactors LD1 and LD4, LD3 and LD6 and LD5 and LD2 respectively. The main thyristors of respective chopper circuits are sequentially turned ON and OFF for 60 electrical angles at a timing shown in FIG. 2 by a control signal having a frequency of 1/6 of that of the output alternating current. The current is controlled by controlling the ratio of ON and OFF periods of the chopper circuits of the same phase during a chopper period of 60.degree.. As it is possible to control the magnitude of the output current and the frequency thereof by the inverter apparatus itself the DC source may be a constant voltage source. Accordingly, where direct current is obtained from an AC source having a commercial frequency the rectifier can be constituted by diodes, whereby it is possible to improve the power factor of the source and to increase the speed of current control by choppers.
However, the inverter apparatus shown in FIG. 1 has the following defects. Namely, when the main thyristor M1 and the phase selection and circulating thyristor S1, for example, are OFF, the voltage at the U phase terminal is caused to become lower than the voltage of the negative bus bar by the induced voltage of a load motor, so that the commutation capacitor is overcharged by a circuit extending through auxiliary commutation thyristor of the reverse conduction type. Accordingly, it is necessary to increase the breakdown voltage of the auxiliary commutation thyristors and the main thyristors. Furthermore, each unit inverter requires a phase selecting and circulation thyristor in addition to the main thyristor and the auxiliary commutation thyristor, so that it is more expensive than a McMaley type inverter commutated by an auxiliary impulse.